Cant pinpoint the problem

For the last 2-3 months I've been having an issue with my mouse it'll randomly freeze and and then windows makes the sound it makes when a usb device is connected and it'll continue working, annoying but I was putting it down to the mouse being nearly 6 years old and possibly on its way out but I tested it on another system and couldn't get it to happen so that stumped me a little.

Within the last month a new symptom (or it may be completely unrelated) has surfaced both my monitors will go off like they were going into sleep mode then return a few seconds later, happens alot when opening programs but not so much when actually using something but will often come at the same time as the aforementioned mouse issue.

Finally much less frequently and with no noticeable trigger my computer likes to restart itself, mid game or while watching a movie is when i notice it but it has happened when its idle too. It doesn't happen very often possibly once every 2 weeks, I've put this down to something forcing a restart following an update such as McAfee but thought I'd include it encase it is actually a contributing factor.

Since I've tested the monitors and the mouse on other systems and they work fine I know its not them, I'm thinking its either a overheating issue (though it doesn't seem to be running hot), a power issue or my motherboard is starting to die, any ideas?

I had this years back and it stemmed from a wrong combination of motherboard and GPU. You can go ages without noticing it, but then it suddenly gets worse.
Second option could be that your RAM is faulty. Sadly all things that must be tested with another PC. Maybe your local shop can help you out if you don't have access or aren't tech savvy.

For the mouse; assuming the issue is not connected to the other two, try using a different USB port. The port may be bad, especially if it's USB 3.0.

If the monitor issue is connected to the mouse, it may well be your motherboard acting up; disconnecting your monitor every now and again, just like the mouse. Otherwise, I don't have a lot of ideas on what it could be.

The random restarts could be caused by over-heating, depending on how your BIOS settings are set up. Checking the temperatures couldn't hurt.